Roadway repair is often accomplished by overlaying the existing pavement (whether of concrete or asphalt paving material) with a new layer (often called a leveling course) of concrete or asphalt paving material. This is an expensive and time-consuming process and is not always necessary. An alternative is to employ a chip sealing process for extending the useful life of the existing roadway. The typical cost of a chip sealing process is 20-25% of the cost of a conventional asphalt overlay. In the chip sealing process, a binder composition including asphalt cement is applied to the surface of the existing roadway, and then a layer of stone chips having high durability and high fracture plane characteristics is spread over the asphalt composition.
A chip seal is designed to seal and protect a pavement from oxidation and weathering by sealing the small cracks in a pavement surface and thereby preventing the intrusion of water into the pavement surface. A chip seal can also be used to address raveling (loss of aggregate), to correct bleeding and flushing problems, to improve skid resistance and generally to extend the pavement life. Chip seals are not suited for all pavements; the existing structure should be sound, with only minor surface defects. Consequently, chip seals are typically applied to pavements that are in relatively good condition with no underlying structural damage. When a chip seal is applied to a pavement in relatively good condition, an additional six to eight years of service may be expected. For more distressed pavements, less service extension can be expected (four to six years on pavements in fair condition and three to four years for pavements in poor condition).
Different types of asphalt cement binder compositions can be used in a chip sealing process. The composition may be applied with or without heating. Asphalt cement binder compositions that are applied without heating in conventional chip sealing operations include modified and unmodified asphalt cement emulsions. Compositions that are applied hot in conventional chip sealing operations can include polymer-modified asphalt cements, polymer-modified asphalt cements blended with rubber or crumb rubber and unmodified asphalt cements. The appropriate binder type is selected based on pavement condition, climate, aggregate properties, and desired service life and cost considerations.
The amount of binder that is applied must be carefully controlled to obtain optimal results. It is desirable to use sufficient binder to bond the chips to the roadway surface, but not so much that the binder bleeds to the upper surface of the roadway over time. When the binder comprises a paving grade asphalt cement, it is difficult to apply enough binder to hold the chips in place on the roadway without significant bleed over. This allows the asphalt cement to be tracked by the wheels of the vehicles that are passing along the roadway. Consequently, in many conventional chip sealing processes, an emulsion is formed of asphalt cement and about 30% water. This emulsified mixture is then applied to the road using an asphalt emulsion supply truck equipped with a spray assembly. However, when the binder comprises an asphalt cement emulsion, the water in the emulsion begins to evaporate as soon as the emulsion meets the road surface. Furthermore, even if the chips are applied before much water has evaporated, the emulsion will tend to settle to the surface of the roadway, leaving much of the surface area of the chips without any binder material. Subsequent vehicular traffic will tend to dislodge the chips from the thin layer of emulsion on the roadway, resulting in an inferior and short-lived repair. One solution to these challenges is to apply a second layer of asphalt cement emulsion (i.e., a fog coat) over the completed chip-sealed roadway.
It would be desirable if a chip sealing method and apparatus could be developed that would avoid the problems associated with conventional chip sealing operations.